1. Field of the Invention
The present invention relates to a photoconductive composition and a display device adopting a photoconductive layer made of the composition, and more particularly, to a photoconductive composition for a photoconductive layer by an electrophotographic technique and a display device adopting a photoconductive layer made of the composition.
2. Description of the Related Art
A phosphor screen of a color cathode ray tube (CRT) is formed by a slurry coating method or an electrophotographic process.
According to the slurry coating method, a panel is cleaned, and then slurries of primary colors (i.e., red, green and blue) emitting phosphors are respectively coated on the panel. Each phosphor slurry contains polyvinylalcohol, one of the red-, green- and blue-emitting phosphors, and ammonium dichromate. After exposing a predetermined portion of the panel to light using a shadow mask, a developing process is performed to form a phosphor screen in a dotted or striped pattern.
However, the slurry coating method has the following problems.
First, the phosphor remains at an unexposed portion after the exposing and developing processes, so that the remaining phosphor is mixed with a phosphor to be coated later.
Second, a coloring substance is generated by the reaction between the hydroxy group of polyvinylalcohol and ammonium dichromate which are contained in the phosphor slurry, thereby lowering purity in color.
As Another method for manufacturing the phosphor screen for CRT, a method using an electrophotographic technique is known, The method is simple in process compared with the above-described slurry coating method, and can provide a color CRT having excellent luminescent characteristic.
In this method, a conductive layer is first formed on a CRT panel which has been cleaned, and a photoconductive layer is formed thereon.
Then, the photoconductive layer is electrified using a corona charger, and a predetermined portion thereof is then exposed through a shadow mask. After neutralizing electric charge of the exposed portion, one of red, green and blue phosphor compositions is adhered to the exposed portion thereof and then fixed to the inner surface of the panel. Then, by repeating the above steps, the remaining phosphor compositions are fixed on the inner surface of CRT panel, respectively, thereby completing a phosphor layer pattern
On the other hand, a plasma display device is for displaying an image using a gas discharge phenomenon. Since the plasma display device are excellent in display capacity, luminance. contrast an viewing angle properties, the plasma display device has been highlighted as a display device capable of replacing the CRT. In the plasma display device, gas discharging occurs between the electrodes by a DC or AC voltage applied to the electrodes, and the phosphor is excited by the accompanying ultraviolet rays' emission, thereby emitting light.
The plasma display device is classified into two types according to a discharging mechanism: alternative current (AC) type and direct current (DC) type.
FIG. 1 is a schematic exploded perspective view showing the structure of a conventional AC type plasma display device.
Referring to FIG. 1, a first electrode 13a as a display electrode, and a second electrode as an address electrode are formed between a front substrate 11 and a rear substrate 12. Here, a plurality of first electrodes 13a and a plurality of second electrodes 13b are formed on the inner surfaces of the front substrate 11 and the rear substrate 12, respectively, with a stripped shape, crossing each other at a right angle.
A dielectric layer 14 and a passivation layer 15 are formed in sequence on the front substrate 11 having the first electrodes 13a. Also, a dielectric layer 14' is formed on the rear substrate 12 having the second electrode 13b, and a plurality of barrier wall 17 are formed on the dielectric layer 14.
A plurality of cells 19 are formed between the barrier walls 17, and the cells 19 are filled with an inert gas such as argon (Ar). Also, a phosphor screen 18 is formed at a predetermined portion of the cells 19.
In the above-described plasma display device, the barrier wall 17 is formed by a printing method where a material paste for the barrier wall is repeatedly deposited on the dielectric layer 14 formed on the rear substrate 12 using a blade coater. However, forming the barrier wall by the printing method causes the following problems.
First, the printing process of the barrier wall materials using the blade coater must be repeated so as to obtain a barrier wall having a predetermined thickness. That is, it takes a long time, lowering productivity.
Second, when coating the paste on the dielectric layer formed on the rear substrate and pressing the resultant structure using the blade coater, a screen mesh attached on the substrate is deformed by the pressure applied by the blade coater. If the screen mesh is deformed, it is impossible to form the barrier walls according to the designed pattern. That is, the shape of the completed barrier walls is distorted, thereby lowering quality of the image.
According to a method using an electrophotographic technique, as another method of forming the barrier walls of a plasma display device, a dielectric layer is formed on a rear substrate having address electrodes, and then a conductive layer and a photoconductive layer are formed on the dielectric layer in sequence.
After electrifying the surface of the photoconductive layer, a predetermined portion of the photoconductive layer is exposed to ultraviolet rays, thereby forming an electrostatic image.
By attaching composition for the barrier wall, i.e., toner composition, to the electrostatic image of the photosensitive film, the photoconductive layer is developed. During the developing process, the toner composition attached to the electrostatic image is dried, and the toner composition remaining the portion other than the electrostatic image is removed.
After repeating the steps of electrifying and developing the photoconductive layer, the rear substrate is sintered, completing the barrier walls.
A photoconductor as the major component of the photoconductive layer is roughly classified into an inorganic photoconductor and an organic photoconductor.
Generally, the inorganic photoconductor is toxic as well as poor in sensitivity, thermal stability, hygroresistance and durability. Also, the inorganic pnotoconductor results in much residue after the sintering process. In order to solve the problems, research into the organic photoconductor has been actively conducted. The organic photoconductor is lightweight, transparent and easy to sinter. Thus, the organic photoconductor has been mainly used when forming a fluorescent film of a CRT or barrier walls of a plasma display device using the electrophotographic technique.
However, the organic photoconductor has a low electrification potential, and poor charge generating and transmission abilities. Also, after sintering the organic photoconductor, residues also remain, thereby lowering image quality of a display device.